Method of preventing carburization in molten metals



United States Patent tanford, Calif., assignors to North AmericanAviation,

No Drawing. Filed Nov. 23, 1959, Ser. No. 854,601 Claims. of. 75-66) Ourinvention relates to a method of protecting metals from carburization inliquid metal coolants.

Liquid metals have excellent heat transfer characteristics, and areemployed as coolants in industrial processes and in nuclear reactors.The liquid metals will, in service, take up carbon from' varioussources, such as graphite, hydrocarbons and certain carbon-containingsteels. Metal compositions containing carbide-forming elements becomecarburized and damaged when exposed to carbon-contaminated liquidmetals. Heretofore, in order to use stainless steels in sodium systems,elaborate precautions have been necessary to limit the carbonconcentration of the sodium. For example, the Liquid Metals Handbook,Sodium-NaK Supplement ('1955), page 177, states:

The carburization of stainless steel is a cumulative process, and thetotal amount of hydrocarbons that can be added to a system during itslifetime must be restricted on the basis of exposed area and allowabledepth of carburization. In one application, a total of 0.0002 lb./ft. ofexposed steel area has been set as a limit for carbon addition of anykind to the system. Prior removal of trace hydrocarbons from the covergas is important, since, unlike oxygen, removal during operation is notfeasible.

An object of our present invention, therefore, is to provide a method ofprotecting structural metals from carburization.

Another object is to provide a composition for removing carbon from aliquid metal coolant system.

Another object is to provide a method of protecting alloy steels fromcarburization in liquid metal systems.

Still another object is to provide a low cost gettering composition forprotecting stainless steel from carburization in sodium.

The above and other objects and advantages of our invention will becomeapparent from the following detailed description taken together with theaccompanying claims.

In accordance with our present invention, metals may be protected fromcarburization in a liquid metal system containing carbon by placing insaid system a metal having a low carbon activity. 7

The materials which may be used in our invention as getters forprotecting all-0y steels from carburization are those metals which havea lower carbon activity than iron. Metals with low carbon activity havea strong affinity (high driving force) for carbon and form stablecarbides relatively rapidly. Iron itself is substantially neutral withrespect to carbon in carbon-saturated sodium.

It has been found that the carbon activity level, rather than the carbonconcentration, is the controlling factor in carbon diffusion. Thus,between steels initially satuice iron, and which may be satisfactorilyused in our invention for carbon gettering, and for protecting alloysteels from carburization, are manganese, chromium, molybdenum,tantalum, vanadium, niobium, and titanium. These elements willpreferentially form more stable carbides than iron.

Our gettering compositions operate effectively in all liquid metals andare not limited to use in any single liquid metal or eutectic system.Examples of the liquid metals are sodium, lithium, tin, bismuth, andbismuthtin.

The foregoing low carbon activity metals may be used singly or incombination for service in gettering carbon and inhprotecting alloysteels from carburization. We found, however, that surface layers ofcarbide or oxide form on the pure metal or metal combinations which actas diffusion barriers, and tend to progressively decrease theeffectiveness of the gettering action. We have discovered that sucheffects can be avoided by distributing the gettering element in adiluent material. This allows further diffusion of carbon into thecomposition, formation of surface carbide diffusion barriers is avoided,and the gettering action is not impaired with the passage of time. As anexample of such behavior, we found that chromium metal, which has a verylow carbon activity, is not as effective in protecting alloy steel fromcarburization as is chromium alloy.

The criteria for a satisfactory base or alloying material include afairly high rate of-c'arbon diffusion therein, solubility for carbon andgetter, and a neutral or slightly lower activity relative to carbon. Thesolubility requirement is important because if a continuous carbidelayer is formed on the surface of the getter, we have found that itforms a diffusion barrier to further absorption of carbon, and hence theeffectiveness of the getter would rapidly diminish after a short time.Among the practical, commercially-available diluents are iron andnickel; alloys of these metals with the low carbon activity metalscomprise useful hot trap carbon getters.

However, even in a diluent material, the effective range of the alloyconstituents varies with concentration. Below about 5 weight percenttotal alloy additive concentration, carbon gettering is low; betweenabout 5-30 weight percent total alloy in either nickel or iron basegetters, efficient carbon gettering is achieved; but beyond about 30weight percent the effectiveness again drops. The drop off beyond 30Weight percent may be due to the formation of nndiffusible surfacelayers of metal carbides, or to the formation of intermetallic compoundsrated with respect to carbon, carbon diffuses down an which act asabarrier to carbon diffusion to the interior. For example, theeffectiveness of chromium alloys as hot trap carbon getters increaseswith increasing chromium content and then decreases. The optimumchromium range within the 5-30 weight percent range is about 912 weightpercent, and a very effective alloy is a 9 Cr-l Mo ferri-tic steel.

The effective ranges of the low carbon activity metals in iron andnickel base alloys are indicated as follows. The effectiveness andcomponent ranges of our gettering compositions are not significantlyinfluenced by the particular liquid metal from which carbon is beingremoved, or by the diluent base.

The alloy constituents, Within the above concentration Remainderessentially Fe Quarternary Alloys Remainder essentially Ni QuaternaryAlloys Nb 0.5-10 V 0.5 V 0.5- Ta 0.5- 5 MO 0.5-10 Ta 0.5- 5 CI 0.5 MO0.5- 5. Ti 0.5-10 Ti 0.5- 3 v 01s- 5 Mo 0.5- 3 Ta 0.5-10 Mo 0.5- 3 V0.5- 5

Remainder essentially Fe Our gettering material may very satisfactorilybe used in a hot trap in an operating liquid metal system. The hot trapwould be of conventional design and contain thin sheets or ribbons ofthe getter in a large surface-tovolume ratio. The material is fabricatedin the form of a cartridge which can be inserted and removed from thehot trap. There could be full liquid metal flow through the hot trap,or, more likely, the hot trap would be situatedin a bypass loop of thesystem. The temperature in the hot trap is desirably maintained at ahigher temperature than the liquid metal system, for instance about 150-400 F..higher, in order to hasten the rate of reaction with the gettermateriaL. In a sodium-cooled nuclear power reactor, the current designsof which operate at peak temperatures of about 1000 F., the side streamhot trap getter would, for example, satisfactorily operate at about 1200F.

The following examples are ofiered to illustrate our carbon getteringinvention in greater detail. Metal samples 0.060 inch thick were exposedfor 200 hours in 1300 F. sodium saturated with carbon. The metals andtest results are shown below.

spaaeeo e 3 4 range for each metal, may be used in any combination inbinary, ternary, and quaternary iron or nickel base alloys, Carbonylcarbon in provided the total alloy content does not exceed about MammalPercent) ggg g gg igggg weight percent. Examples of binary alloys wouldbe any (wt. Percent) (wt. Percent) of the foregoing metals in theindicated concentrations 111 5 iron or nickel. Examples of ternary andquate nary (1L5 (lg-0.551%; .25 r O. 0- nickel and non alloys weightpercent) are as follows. 225 CH Mom 0.12 1. a as ran-:2 5 r-0. 0- 1Ternary Alloys 10 5 (Ir-0.5 Mo-Si 0 o 1-1. Cr 0.54 M 5 lgi i ifiifj 8:13litfi M0 5 5 Ta 0,5 15 304 stainless stce 0.06 0. 72-0.

12 Or (410 stainless steel). 0.10 1. -1. Cr T1 5 1? 8r 430 stainlesssteel) o. 1.

r 0. 0. 7 Ta 0.5 25 V -5 3 or nil OOHM M0 -5-1 T3- 15 Z 11 O Q5-{) (]GRemainder essentially Ni ff 8-8; 5:3 Ternary A [lays LgRtjmaiu deressentially Fe) 0 03 1 08 I I t 21.6 (Dr-2.2 M0 0. 07 1.8 Mo 05-10 V05-10 11.5 gr1.3 %o-l.l Ti 10.3 r-0.5 1 0 Ta 8'258 t 8? 3 a; a; C1- a 2.o- .5 a5. Cr Mo 5 5 (Remainder essentially Ni) V 0.5-10 Ta 0.5-10

It should be appreciated that the foregoing examples are illustrativerather than restrictive of our invention. Our invention should beunderstood, therefore, as being limited only as is indicated in theappended claims.

We claim:

1. A method of preventing carburization of an alloy steel in a hotmolten metal system selected from the group consisting of sodium,lithium, tin, bismuth, and bismuth-tin, which comprises placing in saidmolten metal an alloy consisting essentially of approximately 5-30weight percent of at least one metal selected from the group consistingof manganese, chromium, molybdenum, tantalum, vanadium, niobium, andtitanium, and the remainder at least one metal selected from the groupconsisting of iron and nickel, thereby gettering carbon from said moltenmetal in said alloy.

2. The method of claim 1 wherein said first-named metal in said alloyconsists of about 9-12 weight percent chromium.

3. The method of claim 1 wherein said alloy is a ferritic steelcontaining about 9 Weight percent chromium and about 1 weight percentmolybdenum.

4. The method of claim 1 wherein said alloy, in a form having a highsurface are'a-to-volume ratio, is placed in a hot trap, said trap beingmaintained at a temperature of about ISO-400 F. higher than the portionof said liquid metal system outside of said hot trap.

5. The method of preventing the carburization of an alloy steelstructural material in a liquid sodium system containing carbon at atemperature of about 1000 R, which comprises placing a ferritic steelcontaining about 9 weight percent Cr-1 weight percent M0 in a highsurface area-to-volume ratio in a side stream hot trap maintained at atemperature of about 1200 F., and passing said sodium through said hottrap, thereby gettering carbon on said ferritic steel.

References fiited in the file of this patent UNITED STATES PATENTSSibert Dec. 21, 1954 Olsen Nov. 12, 1957 Batutis et a1 Mar. 24, 1959OTHER REFERENCES Bain: Alloying Elements in Steel, A.S.M. 1939, page 75relied on, American Society for Metals, Cleveland, Ohio.

Brick and Phillips: Structure and Properties of Alloys, 2d edition,McGraw-Hill, New York, 1949, page relied on;

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,043,680 July 10, 1962 Walter C. Hayes et al.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 3, lines 9 to 50, the table should appear as shown below insteadof as in the patent:

TERNARY ALLOYS 5-25 Cr; 0.5- 5 Mo Remainder essentially Ni 5- 5 M;0.5-15 Ta -25 Cr; 0.5- 5 Ti 5-25 Ta; 0.5- 3 V 5-10 M0; 0.5-15 Ta 5-1OM0; 0.5- V Remainder essentially Fe 5-20 Ta; 0.5- 5 Ti 5-20 Cr; 0.5-10Ta 52O Cr; 0.5- 5 M0 5-10 V 0.5-10 Ta 0.5- Cr; 0.5- 5 Mo; 0.5-10 TaRemainder essentially Ni 0.5- 5 M0; 0.5-10 Ta; 0.5- 5 Ti 0.5-15 Cr; 0.5-5 M0; 0.5- 3 Ti 0.5-l5 Cr; 0.5- 5 M0; 0.5- 3 V 0.5-15 Mn, 0.5-10 Ta;0.5- 3 Ti O.5*-10 T1 0.5-10 Ta; 0.5- 3 V 0.5-15 Cr 0.5- 3 Ti 0.5- 3 VQUATERNARY ALLOYS O.5-l0 Nb; 0.5- 5 V 0.5- 5 Ta Remainder essentially Fe0.5-10 V 0.5-10 Ta; 0.5-- 5 M0 0.5-10 M0; 0.5- 5 V 0.5- 3 Ti 0.5- Cr;0.5- 5 V 0.5- 3 M0 0.5-l0 Ti; 0.5- 5 Ta; 0.5- 3 M0 Signed and sealedthis 29th day of January 1963. (SEAL) Attest ERNEST W. SWIDER DAVID LLADD

1. A METHOD OF PREVENTING CARBURIZATION OF AN ALLOY STEEL IN A HOT MOLTEN METAL SYSTEM SELECTED FROM THE GROUP CONSISTING OF SODIUM, LITHIUM, TIN, BISMUTH, AND BISMUTH-TIN, WHICH COMPRISES PLACING IN SAID MOLTEN METAL AN ALLOY CONSISTING ESSENTIALLY OF APPROXIMATELY 5-30 WEIGHT PERCENT OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF MANGANESE, CHROMIUM, MOLYBDENUM, TANTALUM, VANADIUM, NIOBIUM, AND TITANIU, AND THE REMAINDER AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF IRON AND NICKEL, THEREBY GETTERING CARBON FROM SAID MOLTEN METAL IN SAID ALLOY. 